1. Field of the Invention
The invention relates generally to radar system test equipment and more specifically to equipment for evaluating moving target indicator radar performance as a function of transmitted pulse stability.
2. Description of the Prior Art
MTI Radar performance is frequently specified in terms of Improvement Factor I, which is defined as the ration of output target-to-clutter power to input target-to-clutter power, averaged over all target speeds. This precise definition used as a figure of merit for MTI systems takes into account the average gain of the MTI system and the clutter attenuation properties. The response of an MTI system to clutter relative to average MTI system response to bona fide targets is thereby determined. The value of I obtained by a system is a function of the clutter environment. Modern radar specifications usually include specific clutter models to be used in evaluation of system performance. For example, a high performance search radar might be required to have Improvement Factors on the order of 60 dB against ground clutter and 35 dB against specified rainfall rates. Landing system requirements may specify Improvement Factors on the order of 35 dB against ground clutter.
Instabilities in the radar transmitter, receiver, and processor have the effect of disturbing the phase relationships in a coherent radar or, equivalently, increasing the spectral spread of the clutter. These instabilities thus degrade MTI performance, and specification and control of the stability of various signals in the radar becomes important.
In connection with such systems, much work has been done characterizing the frequency stability parameters of the CW oscillators used in radar systems to provide the local oscillator (STALO or STAMO) and the coherent oscillator (COHO). Several studies have also related these parameters to system performance. Radar textbooks and reference books (for example the text, RADAR HANDBOOK, by Merrill J. Skolnik, McGraw Hill, 1970) relate the various system instabilities to system performance.
The aforementioned investigative work allows measurements to be made at the subsystem level and to be related to system performance. This measurement at subsystem level has obvious advantages in that design parameters can be specified, optimized, and measured for required performance and system performance can be more accurately assessed before final consolidation of the system. In this way, inadequate system performance can be localized to specific problems in subassemblies, which can be upgraded as necessary. While the relationships betwen system performance and subsystem measurements cannot completely substitute for final system evaluation, they can certainly minimize the risk involved in the overall design.
Radar transmitter subsystems may be either of the oscillator (magnetron for example) or MOPA type (ending in a power amplifier state such as an amplitron).
The types of instabilities of transmitting subsystems which contribute to MTI radar performance deterioration, and therefore, require careful evaluation, are of several types. Among the most important of these are timing, pulse width, amplitude, phase and frequency.
The technical literature variously relates system performance in transmitter instabilities in connection with the so-called Improvement Factor hereinbefore mentioned. In the text, "Radar Design Principles" by F. E. Nathanson (Mc-Graw Hill, 1969), Chapter 9, an analysis is given from which the improvement factor can be written as follows: ##EQU1## where CA = Clutter attenuation
S.sub.i = Input signal PA1 S.sub.o = Output signal averaged over all target velocities
Investigation of the above relationship for each type of transmitter instability recognized allows development of a set of equations relating measurements of the variance, first difference or pulse-to-pulse variance (single canceller), or second difference variance (double canceller) of each fluctuation, to system Improvement Factor limitations. When the disturbances are substantially uncorrelated, then the Improvement Factor I may be written as a total Improvement Factor from all the uncorrelated disturbances I.sub.T, as follows: N being the number of difference causes considered: ##EQU2##
A compilation of Improvement Factor limitations due to the various instabilities in transmitters is given in Table I, assuming that the various disturbing factors are uncorrelated.
TABLE 1 __________________________________________________________________________ SYSTEM IMPROVEMENT FACTOR LIMITATIONS DUE TO TRANSMITTER INSTABILITIES IMPROVEMENT FACTOR (I).sub.dB = 10 LOG[ ] MEASUREMENT FIRST DIFFERENCE SECOND DIFFERENCE VARIANCE VARIANCE VARIANCE __________________________________________________________________________ TIMING INSTABILITIES .tau..sup.2 .tau..sup.2 SINGLE CANCELLER -- 2.sigma..sub.t.sup.2 .sigma. .sub.t.sup.2 .tau..sup.2 3.tau..sup.2 DOUBLE CANCELLER -- 2.sigma..sub.t.sup.2 .sigma. .spsb.2.sub.t.sup.2 PULSEWIDTH INSTABILITIES .tau..sup.2 2.tau..sup.2 SINGLE CANCELLER -- .sigma..sub..tau..sup.2 .sigma. .sub..tau..sup.2 .tau..sup.2 6.tau..sup.2 DOUBLE CANCELLER .sigma..sub..tau..sup.2 .sigma. .spsb.2.sub..tau..sup.2 9 AMPLITUDE INSTABILITIES A.sup.2 2A.sup.2 SINGLE CANCELLER -- .sigma.A.sup.2 .sigma. .sub.A.sup.2 A.sup.2 6A.sup.2-DOUBLE --NCELLER .sigma..sub.A.sup.2 .sigma. .spsb.2.sub.A.sup.2 FREQUENCY INSTABILITIES (OSCILLATORS) 3 6 SINGLE CANCELLER -- (2.pi..tau.).sup.2.sigma..sub.f.sup.2 (2.pi..tau.).sup.2.sigma. .sub.f.sup.2 3 18 DOUBLE CANCELLER -- (2.pi..tau.).sup.2.sigma..sub.f.sup.2 (2.pi..tau.).sup.2.sigma. .spsb.2.sub.f.sup.2 PHASE INSTABILITIES (AMPLIFIERS) 1 2 SINGLE CANCELLER -- .sigma..sub..phi..sup.2 .sigma. .sub..phi..sup.2 1 6 DOUBLE CANCELLER -- .sigma..sub..phi..sup.2 .sigma. .spsb.2.sub..phi..sup. 2 WHERE .sigma..sub.( ).sup.2 = VARIANCE OF INDEPENDENT SAMPLES OF FLUCTUATION .sigma. .sub.( ).sup.2 = FIRST DIFFERENCE VARIANCE .sigma. .spsb.2.sub.( ).sup.2 = SECOND DIFFERENCE VARIANCE .tau. = PULSEWIDTH A = AMPLITUDE __________________________________________________________________________
With the foregoing as background, it will be realized that the analysis of a radar system requires the accurate measurement of the net effects of various transmitter pulse instabilities through the entire signal processing chain of a radar system. In the prior art, efforts have been exerted to measure the instabilities themselves and their effects. A computing counter, such as manufactured by the Hewlett-Packard Company under the designation HP5360, along with associated hardware, has been used to this end. Such a device however, is inherently limited to low pulse repetition rates in a pulse-to-pulse RMS mode.
A need clearly exists for a test set for the specialized MTI performance evaluation problem so that a much wider range of repetition rate accommodation is possible, and for a "net effect" display presented as a function of the instability of each parameter to be evaluated.
The manner in which the device of the present invention provides a device capable of MTI system instability measurement over a broad range of repetition rates (and even for staggered PRF) will be evident as this description proceeds.